Conventional pressure diffusers, such as those sold by Kamyr, Inc. of Glens Falls, N.Y. under the trademark "MC.RTM.", are typically one stage. Some partial two stage units--attempting to gain the advantages for multi-stage units often achieved in atmospheric diffusers--are in use with no interval separation between stages, and are relatively inefficient. Attempts at full two-stage operation, or variable stage operation, have not been commercially successful to date.
In view of the fact that the pressure diffusers operate at a pressure that is always substantially greater than one atmosphere throughout passage of the pulp being treated through the vessel, there have been perceived impediments to multi-stage configurations. Perhaps the largest perceived impediment in conventional thinking is that there is no good way to divide the flow without resulting in channeling of the pulp, or plugging of the screens, and that mixing between the extraction flows of the different stages would occur thereby destroying the benefits of a multi-stage approach.
In the parent application, a practical two stage pressure diffuser was provided. While that diffuser is eminently suitable for situations where two stages are practiced, there are many circumstances where it is desired to be able to vary the number of stages. For example sometimes it is desirable to operate the diffuser on less than two full stages (such as 1 3/4, 1 1/2, or 1 and 1/4 stages), and the pressure diffuser in the parent application does not practically allow such practice. Therefore, according to the invention modifications have been made to the pressure diffuser in the parent application in order to allow variable stage operation in a practical way. The primary modifications that have been made are to provide an open bottom for the tubular baffle of the diffuser in the parent application, and to provide a pair of vertically spaced interior headers located within the baffle, and connected by conduits to extraction arms at the top of the vessel. Also, treatment liquid may be introduced at more than two different points along the height of the diffuser vessel.
According to one aspect of the present invention, a screen and baffle assembly for a treatment vessel is provided. Such an assembly comprises: An annular screen assembly elongated in a first dimension about an axis and having first and second ends, and defining an interior volume. Bearing means disposed at the first end of the screen assembly. Connection means disposed at the second end of the screen assembly, adapted to connect the screen assembly to a power source. An inner hollow cylinder mounted within the screen assembly and elongated in the same dimension of elongation as the screen assembly, and coaxial therewith and having a first end and a second end. The inner cylinder and the screen assembly bearing means engaging at the first end of each. The cylinder having a length roughly about 55-75% of the screen assembly length. And, means defining a first plurality of radially extending openings in the cylinder adjacent the second end thereof, and communicating with a first header disposed interiorly of the inner cylinder, with a first conduit extending from the header to the exterior of the vessel. The assembly also further comprises means defining a second plurality of radially extending openings in the cylinder between the first and second ends thereof, and communicating with a second header disposed interiorly of the inner cylinder, with a second conduit extending from the second header to the exterior of the vessel. First, second, and preferably third (or more) liquid treatment introduction means are provided vertically spaced along the vessel. As is conventional, the screen assembly is reciprocated up and down within the vessel.
Also, according to the invention a separate sealing means may be provided at the end of the cylinder, engaging the screen assembly, to prevent mixing between stages. The sealing means may be an annular element of elastomeric material, such as ethylene propylene rubber, with an annular skirt biased into engagement with the screen. This sealing means may also be provided on the cylinder in the two stage assembly of parent application Ser. No. 07/685,400 filed Apr. 16, 1991.
According to another aspect of the present invention, a pulp treating apparatus is provided which comprises the following elements: A generally upright, liquid-tight, pressurized vessel defining a first interior volume for containing pulp to be treated under pressure. A pulp inlet to the vessel. A pulp outlet from the vessel, the pulp flowing generally vertically between said pulp inlet and the pulp outlet. A screen assembly defining a surface of revolution upstanding within the vessel and defining, in part, the first interior volume containing pulp. Extraction means for withdrawing liquid from the pulp, through the screen, and including means defining a second interior volume within the vessel for receiving the extracted liquid, the extraction means comprising first, second and third extraction mechanisms vertically spaced from each other within the second interior volume, for extracting liquid at three different points within the second interior volume. First treatment liquid introduction means for introducing a first treatment liquid into the first interior volume. And, second treatment liquid introduction means vertically spaced from the first treatment liquid introduction means, and for introducing a second liquid, distinct from the first liquid, into the first interior volume.
The apparatus further preferably comprises headers disposed within the interior of the screen assembly, with first and second conduits extending from the headers to the exterior of the vessel; and wherein the third extraction mechanism comprises a conduit in communication with the second interior volume, the conduit at the bottom of the vessel. The apparatus also desirably includes an interior cylinder mounted within the surface of revolution screen assembly and radially spaced therefrom, and means defining a plurality of openings in the cylinder adjacent each of the first and second headers so that extracted liquid may freely flow into the headers through the cylinder at the openings, the cylinder having an open bottom so that the interior of the cylinder is in communication with the second interior volume, and third extraction mechanism; and a third liquid introduction means is also preferably provided.
According to yet another aspect of the present invention a method of treating comminuted cellulosic fibrous material suspension is provided. The method comprises the steps of: (a) Introducing the suspension under pressure into one end of the vessel so that it flows vertically within the first interior volume, the suspension being maintained at a pressure substantially greater than one atmosphere throughout its passage in the vessel. (b) Extracting liquid through the screen assembly into the second interior volume, the liquid being withdrawn from the suspension in the first interior volume. (c) Introducing a first treatment liquid through the circumferential side walls of the vessel at a first vertical portion of the vessel. (d) Introducing a second treatment liquid, distinct from the first treatment liquid, at a second vertical portion of the vessel distinct and vertically spaced from the first vertical portion of the vessel. (e) Withdrawing the extraction liquid from three different vertical portions within the vessel. And, (f) reintroducing some of the withdrawn liquid from step (e) as the second treatment liquid.
It is the primary object of the present invention to provide a variable pressure diffuser, and method of pressure diffusion treatment (e.g. washing) of cellulosic pulp using such a device. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.